The present invention relates to analytical chemistry and, more particularly, to a particle-beam generator, for example, to an interface between a liquid chromatography system and a mass spectrometer. A major objective of the present invention is to provide for more effective nebulization of liquid chromatography effluent having a high aqueous content.
LC/MS systems, which combine liquid chromatography (LC) and mass spectrometry (MS), are used for several purposes including 1) environmental studies, for example, to evaluate water, soil and waste; 2) food analysis, to identify contaminants and adulterants; 3) pharmaceutical development, to analyze natural and synthetic products; and 4) life sciences, to characterize protein components.
Liquid chromatography is a method of separating components of a sample mixture. At any given time during separation, some molecules of a component are adsorbed to a stationary solid support, while other molecules are dissolved in a liquid solvent flowing past the solid support. The adsorbed molecules are said to be in a "stationary phase" while the dissolved molecules are said to be in a "mobile phase". Sample components can differ significantly in their solubility in a given solvent. Specifically, nonpolar components tend to dissolve more readily in organic solvents, while polar components tend to dissolve more readily in water. To accommodate samples with both polar and nonpolar component, reverse-phase gradient-elution liquid chromatography (GELC) provides for a gradual transition of organic solvent to water as the liquid solvent in an LC system.
At equilibrium, the rate at which a component's molecules in the stationary phase are released to the mobile phase equals the rate at which the same component's molecules in the mobile phase are adsorbed to the stationary phase. For each component, the ratio of the number of molecules in the stationary phase to the number of molecules in the mobile phase is quantified by a partitioning coefficient. This partitioning coefficient thus corresponds to the average percentage of time the molecules of a component are in the mobile phase. This percentage correlates with the mobility of the component past the solid support. Sample components with different mobilities separate, as they progress past the solid support. With sufficient separation, the components emerge serially in the chromatography effluent.
To complete the analysis of a sample mixture, the eluting components need to be identified and quantified. Various types of detectors, for example, ultra-violet absorption detectors positioned to monitor the ultraviolet absorption characteristics of the effluent, can be used to detect eluting components. Since each component has a characteristic retention time in a chromatographic column, the time of detection is often used for component identification, while the degree of ultraviolet absorption can be used to quantify the component.
However, it is often not possible to identify and quantify sample components dissolved in the chromatography effluent. Some components are not readily detectable, others appear in quantities too small to measure reliably, and others can not be uniquely identified by their retention times. In these situations, and others, a mass spectrometer can be used for sample component identification and quantification.
A mass spectrometer provides a mass spectrum of a sample component by separating sample subcomponents according to molecular mass and quantifying the number of subcomponent molecules at molecular mass. (The samples input to the mass spectrometer are the serialized components of the sample input from the LC system.) Mass spectrometers typically operate by ionizing sample molecules and then sweep-filtering the resulting ions according to their charge-to-mass ratios. To minimize interference with ion movement through the mass filter, mass spectrometers are operated under vacuum conditions.
The liquid output of the LC system is not directly compatible with the requirements for ionization and the vacuum conditions of the mass spectrometer. Accordingly, LC/MS interfaces can include a particle-beam generator that converts a liquid flow into a particle beam. A typical particle-beam generator comprises a nebulizer, a desolvation chamber, a momentum separator, and a transfer probe. In the nebulizer, the LC effluent is joined by a stream of helium and converted into an aerosol of uniform droplets. Solvent is vaporized as the droplets traverse the desolvation chamber, freeing sample particles.
The sample particles proceed as a beam through a momentum separator. Vacuum pumps maintain the momentum separator at a lower pressure than the desolvation chamber. The vacuum pumps divert throughgoing particles laterally, drawing lower momentum helium and solvent vapor into the vacuum exhaust system. The higher momentum sample particles are deflected less and are thus permitted to enter the transfer probe. Particles entering the transfer probe are directed to the ion source of the mass spectrometer.
The efficiency of such a particle-beam generator depends, in part, on the solvent of the liquid input. The particle-beam generator is most effective when the solvent is primarily organic, and less efficient when the solvent is primarily aqueous. The mass spectrometer signal strength can fall by 70% or more when the solvent is more than 50% aqueous. This signal loss is particularly problematic in GELC since it makes it difficult to compare mass spectra from earlier eluting components with those of later eluting samples.
This problem has been addressed by adding organic solvent to an aqueous LC effluent. However, this approach results in unacceptable band broadening. What is needed is a system that efficiently generates a particle beam from an aqueous input without unacceptable band broadening.